1. Field of the Invention
The present invention relates to a quantum interference device utilizing the interference of electron waves, and to a complementary logic circuit constructed by connecting together a plurality of quantum interference devices.
2. Description of the Prior Art
An electron has the nature of a particle and that of a wave. Ordinary semiconductor devices such as diodes, transistors, and integrated circuits comprising them combined together, utilize the nature of an electron as a particle. Recent progresses in crystal growth technologies and microfabrication techniques, on the other hand, have enabled the production of devices utilizing the nature of an electron as a wave. Quantum interference devices that modulate the conductance of the device by controlling the interference of electron waves are being studied as such devices. High expectation is placed on the realization of quantum interference devices as elements with a high mutual conductance and capable of high frequency operation, including ultra-high speed switching, or as analog elements for frequency multiplication.
Quantum interference devices are available as devices having a stub structure or a ring structure. Miller et al. have reported that the conductance of such a device can be modulated by forming a gate electrode on a stub of a T-shaped electron waveguide structure or a quantum wire structure with the stub, and controlling a voltage applied to the gate electrode (D. C. Miller, R. K. Lake, S. Datta, M. S. Lundstrom, M. R. M elloch, and R. Reifenberger; "Modulation of the conductance of T-shaped electron waveguide structures with a remote gate electrode," Nanostructure Physics and Fabrication, Academic Press, Inc. (1989), pp. 165-174).
FIG. 1A is a schematic plan view of the quantum interference device of Miller et al., while FIGS. 1B and 1C are schematic Sectional views taken along the lines B--B and C--C, respectively, of FIG. 1A. As illustrated in FIGS. 1A and 1B, a quantum wire 11 has a T-shaped structure or a stub structure in which semiconductor heterojunctions comprising an undoped-GaAs (ud-GaAs) layer 11a, an undoped-AlGaAs (ud-AlGaAs) layer 11b, an n-AlGaAs layer 11c, and an n.sup.+ -GaAs layer 11d formed on a ud-GaAs substrate 11S are finely processed by etching to a width of 0.30-0.4 .mu.m, and a stub 14 is formed behind a branched portion 16. A gate electrode 15 is disposed above a part of the stub 14. An electron wave 1 entered from an entrance 12 is transmitted through a channel 17 to the branched portion 16, where it is divided into an electron wave 2 going into the stub 14, and an electron wave 4 traveling toward an exit 13. The electron wave 2 is reflected aE the rear end of the stub 14, and returned to the branched portion 16 as an electron wave 3. There, the electron wave 3 and the electron wave 1 interfere with each other. If the phase of the electron wave 3 and that of the electron wave 1 are in phase, the probability of transmission from the entrance 12 to the exit 13 is high. If their phases are opposite to each other, the transmission probability is low. The phase relationship at the time of interference is determined by a stub length, Ls. The stub length Ls is controlled by voltage that is applied to the gate electrode 15 formed above the quantum wire 11. That is, this device performs a 3-terminal operation by imparting a potential difference between the exit 13 and the entrance 12, and applying a control voltage to the gate electrode 15.
In the above quantum interference element of an electric field control type, the transverse mode number contributing to transmission is desirably small in order to obtain satisfactory switching characteristics based on an interference effect. Ideally, a single mode operation which gives a transverse mode number of 1 is desirable.
In the above-described conventional device, however, the quantum wire 11 is formed by etching and have a mesa type structure thus inevitably involving the reflection and interference of electron waves due to tiny irregularities of its side walls, and making it difficult to control the mode number of the channel 17 by controlling the channel width. Hence, a single mode channel with marked interference characteristics is difficult to achieve with this type of device. With the conventional quantum interference device, moreover, the electron concentration (electron wavelength) of the channel cannot be controlled freely, and so the modulation period of conductance cannot be controlled.
S. Bandyopadhyay et al. have proposed a quantum interference transistor (QUIT) which controls conductance by introducing a potential difference between two channels which in turn gives rise to a phase shift between electrons by a control terminal (Proceeding of IEDM, 1986, pp. 76-79). The QUIT of Bandyopadhyay et al. is a quantum interference transistor using a longitudinal ring structure formed by laminating two channels connected together at both ends within a semiconductor substrate. This type of quantum interference transistor has a region with a positive transconductance and a region with a negative transconductance relative to a gate voltage, since conductance generally oscillates as a function of the gate voltage. In the foregoing conventional device, two quantum interference devices of a longitudinal ring structure are connected in series, with one of them being biased in the positive transconductance region and the other being biased in the negative transconductance region. This permits the construction of a complementary logic circuit in which the channel of one of the devices is turned off in a static condition. A quantum interference transistor works at a power source voltage of the order of at most several tens of millivolts, and the realization of a complementary logic circuit enables operation with an ultra-low power consumption.
F. Sols et al. have proposed the construction of a complementary inverter circuit by connecting in series two interference transistors with slightly varied stub lengths which have a quantum wire of a stub-shaped protruding structure and a gate electrode for varying the stub length (F. Sols, M. Macucci, U. Ravaioli, K. Hess; J. Appl. Physics, vol. 66 (1989), pp. 3892-3906).
However, the longitudinal quantum interference ring of Bandyopadhyay et al. was very difficult to produce, because its manufacture required epitaxial growth, followed by the formation of a barrier for separating the upper and lower paths by etching, further followed by regrowth by the epitaxial technique; also, the device had to be produced vertically symmetrically for its operation. Furthermore, the gate voltage had to be biased for device construction, making the device structure and circuit construction complicated.
Sols et al., on the other hand, only made a theoretical investigation, and their paper only described the use of two interference transistors with slightly varied stub lengths in regard to the way of achieving interference transistors complementary to each other. It presented no concrete way of achieving them.